Special-shaped seamless steel pipe is a general term for seamless steel pipes with other cross-sectional shapes other than round pipes. According to the shape and size of the steel pipe section, it can be divided into equal-wall thickness special-shaped seamless steel pipe (code D), unequal wall thickness special-shaped seamless steel pipe (code BD), variable diameter special-shaped seamless steel pipe (code BJ). Shaped seamless steel pipes are widely used in various structural parts, tools and mechanical parts. Compared with the round tube, the special-shaped tube generally has a larger moment of inertia and section modulus, and has a larger bending and torsion resistance, which can greatly reduce the structural weight and save steel.